First webkit2gtk security update for stretch

[Jeremy Bicha]

Hi,

This week, there is a new webkit2gtk point release (2.16.4) fixing
high-impact bugs and a security issue. I would like to get this into
Debian Stretch as a security update.

I have never done a Debian Security update before so please let me
know how I can help with preparing this update.

References
========
[1] https://webkitgtk.org/2017/06/20/webkitgtk2.16.4-released.html
[2] https://security-tracker.debian.org/tracker/CVE-2017-2538
[3] Built on all supported architectures so far for unstable ( still
waiting to build on armel, armhf and mips* )
https://buildd.debian.org/status/package.php?p=webkit2gtk
[4] https://launchpadlibrarian.net/324878602/webkit2gtk_2.16.3-2_2.16.4-1.diff.gz
[5] Short URL for upstream diff: https://is.gd/p8qDBn
[6] https://trac.webkit.org/log/webkit/releases/WebKitGTK/webkit-2.16
(commits are from 218536 to 218589)

Thanks,
Jeremy Bicha